1. Field of the Invention
The present invention relates to a photovoltaic cell having a rear-surface reflecting layer (hereinafter referred to as a back reflector).
2. Description of the Related Art
Hitherto, as a known method for improving the conversion efficiency of a photovoltaic cell, the photoelectric conversion layer of a photovoltaic cell is coated with a metallic reflection layer to efficiently utilize incident light, wherein the metallic reflection layer is provided on a surface of the conversion layer opposite the surface upon which light is incident (hereinafter referred to as light-entering surface). Further, in another known method, a transparent conductive layer is provided between such a metallic reflection layer and the photoelectric conversion layer. According to this structure, diffusion of ingredients in the metallic reflection layer into the photoelectric conversion layer can be prevented, flow of excess current can be prevented as well when electrical shorts occur in the photoelectric conversion layer, and further, adhesion of the photoelectric layer can be improved. Such methods are disclosed, for example, in Japanese Examined Patent Publications No. 5943101, No. 60-41878, and 60-84888. Additionally, Y. Hamanaka, et al., Appl. Phys. Lett., vol. 43, p. 644 (1983), have disclosed the insertion of a TiO.sub.2 transparent conductive layer between a metallic layer and a photoelectric conversion layer.
Moreover, it is also proposed to make the front surface of such a transparent conductive layer finely lumpy, namely, to introduce a so-called textured structure onto the front surface. According to such a structure, light is scattered at the interface between the transparent conductive layer and the photoelectric conversion layer, and as a result, more efficient light-absorption can be achieved. Such a method is disclosed, for example, by H. Deckman Proc. 16th IEEE Photovoltaic Specialist Conf., p. 1425 (1982).
The above-mentioned structures, however, cause some problems concerning processability or durability when a photovoltaic cell is practically manufactured while employing a rear-surface electrode (hereinafter referred to as a back electrode) which has such a structure.
One of them is a problem concerning the so-called textured structure. Hitherto, the textured structure typically comprises a rough surface with pyramidal lumpy patterns as illustrated in Tiedje, et al., Proc. 16th IEEE Photovoltaic Specialist Conf., p. 1423 (1982), since such pyramidal lumpy patterns are considered to exhibit an excellent light-confinement effect. When a transparent conductive layer is formed on such a surface of a substrate, however, the surface of the transparent conductive layer will also be rough with pyramidal lumpy patterns. Due to this, when such a transparent conductive layer is inserted between a back reflector and a semiconductor layer (a component layer of a photoelectric conversion layer), current leakage in a photovoltaic cell through defective parts or the like in the semiconductor layer become large, and therefore, the yield of the photovoltaic cell is frequently reduced. Further, when a semiconductor layer of a photovoltaic cell is formed on a rough surface with pyramidal lumpy patterns, the substantial thickness of the semiconductor layer become thinner than that of a semiconductor layer formed on a specular surface. Accordingly, a doped layer or the like of the photovoltaic cell, which is originally designed to be thin, becomes further thinner, and in some cases, the photovoltaic cell has a lower open circuit voltage (Voc) and a fill factor (FF) as compared to a photovoltaic cell formed on specular surfaces of substrates.
Moreover, a photovoltaic cell which has a metallic back reflector comprising Ag, Cu or the like has been found to undergo migration of Ag, Cu or the like when it is used in a humid atmosphere with a plus bias voltage upon the metallic back reflector, where such migration generates continuity to the electrode on the light-entering side to cause shunting in the photovoltaic cell. This phenomenon is remarkable when the metallic back reflector has a textured structure, namely, a surface which is rough to the same level as the wavelength of light in the layer.
Additionally, in a case where Al is used for the metallic back reflector, although migration occurring in the case where Ag or Cu is used does not occur, the reflectance may be reduced when a textured structure is employed. Especially, in some case where a transparent conductive layer is stacked on an Al layer having a textured structure, the reflectance is severely reduced.
Meanwhile, in practice, substrates having surfaces with the least possible roughness as being nearly specular have been preferred for photovoltaic cells in view of their characteristics and yields. However, when a back reflector or both a substrate and a back reflector of a photovoltaic cell are formed so as to have diffuse reflectance values of 1% or less, the surface of a transparent conductive layer to be formed also becomes smooth. In this case, since light is less scattered on the rear surface of the photovoltaic cell, light-absorption by a semiconductor layer of the photovoltaic cell is insufficient. At the same time, in some combinations of materials for the substrate and the back electrode, adhesion between the substrate and the back reflector, the back reflector And the transparent conductive layer, or the transparent conductive layer and the semiconductor layer is also insufficient, and therefore, peeling readily occurs at any interface between these layers during the manufacturing process of the photovoltaic cell. Additionally, polishing a surface of the substrate so as to be specular requires an increased cost in producing the substrate, and therefore, the cost of manufacturing photovoltaic cells also increases.
The above-described problems markedly occur when a practicably-suitable and low-cost manufacturing process is employed, for example, a low-cost substrate comprising a resin film or stainless steel is used, or the manufacturing rate is hastened by hastening the rate of manufacturing semiconductor layers. The manufacturing yields of photovoltaic cells have been limited to low levels due to such problems.